The formation of various integrated circuit (IC) structures on a wafer often relies on lithographic processes, sometimes referred to as photolithography, or simply lithography. As is well known, lithographic processes can be used to transfer a pattern of a photomask (also referred to herein as a mask or a reticle) to a wafer.
For instance, patterns can be formed from a photo resist layer disposed on the wafer by passing light energy through a mask having an arrangement to image the desired pattern onto the photo resist layer. As a result, the pattern is transferred to the photo resist layer. In areas where the photo resist is sufficiently exposed and after a development cycle, the photo resist material can become soluble such that it can be removed to selectively expose an underlying layer (e.g., a semiconductor layer, a metal or metal containing layer, a dielectric layer, a hard mask layer, etc.). Portions of the photo resist layer not exposed to a threshold amount of light energy will not be removed and serve to protect the underlying layer during further processing of the wafer (e.g., etching exposed portions of the underlying layer, implanting ions into the wafer, etc.). Thereafter, the remaining portions of the photo resist layer can be removed.
There is a pervasive trend in the art of IC fabrication to increase the density with which various structures are arranged. As a result, there is a corresponding need to increase the resolution capability of lithography systems. One promising alternative to conventional “dry” lithography is a next-generation lithographic technique known as immersion lithography. In the immersion lithography systems proposed to date, the wafer to imaged by a lithography system is placed in a flowable liquid medium, through which the patterned light is transmitted. Known immersion lithography mediums are purified de-ionized water for use in conjunction with a 193 nm light source 14 (e.g., an argon fluorine (ArF) laser) and polyfluoroethers for use in conjunction with a 157 nm light source 14. These readily flowable immersion mediums replace an air or gas gap that is conventionally present between the final lens of a conventional dry lithography imaging system and the wafer.
However, attempts to implement immersion lithography have encountered a number of challenges. For example, minor variations and/or nonuniformities in the index of refraction of the immersion medium can adversely affect the quality of the exposure pattern incident on the wafer. The cause(s) of changes in the index of refraction of the liquid immersion medium can include, for example, flow of the immersion medium, changes in density of the immersion medium, changes in temperature of the immersion medium and so forth. In dry lithography, several joules of energy can be absorbed by the wafer from the exposure dose. In immersion lithography, it is believed that at least some of the energy from the exposure dose will be absorbed by the immersion medium. Also, since the immersion medium is in contact with at least the wafer, heat may be transferred from the wafer to immersion medium. The absorption of energy by the immersion medium, even in small quantities, may be sufficient to create a variation and/or nonuniformity in the index of refraction of the immersion medium that could adversely affect the imaging pattern. Also, the wafer may be mounted on a stage that is moved relative to the imaging subsystem. For example, the wafer may be exposed, then moved about 30 mm to a new location and stopped for a second exposure and so forth. Wafer motion rates can be about 250 mm/second to about 500 mm/second. This movement may create non-uniform fluid flow (e.g., turbulence, laminar flow, vortex flow, etc.) or other changes to the properties of the immersion medium that could result in variations in the index of refraction of the immersion medium. In addition, it is contemplated that the immersion medium may be purposefully placed in motion (e.g., in a flow pattern over the wafer) or subjected to hydraulic pressure. These factors may also invoke variations in the index of refraction of the immersion medium.
Bubbles and/or contaminants in a liquid immersion medium can also disrupt imaging of the wafer. Fluid flow of the immersion medium may cause bubbles to form and/or become submersed in the immersion medium. During exposure, gases may be released from the photo resist. These gases could dissolve in the immersion fluid, ultimately leading to bubble formation. It is also contemplated that a foreign body may be carried on the wafer and brought into the immersion medium where the foreign body dislodges from the wafer and begins to “float” in the immersion medium. Liquid immersion mediums are used repeatedly for multiple wafers and are subjected to repeated application of radiation that could lead to a significant build up of bubbles, dissolved matter and/or contaminants over time.
Accordingly, there exists a need in the art for improved immersion lithography processes and systems.